1. Field of the Invention
This invention relates generally to centrifugal separation apparatus and methods concerning separating a mixture comprising at least two different constituents. More particularly, the present invention relates to centrifugal separation apparatuses and methods employing centrifugal force in combination with microwave energy for separating a mixture comprising at least two constituents. For instance, the present invention relates to centrifugal separation apparatus and methods pertaining to separation of mixtures such as a colloidal suspension or dispersion. Optionally, or additionally, a mixture may include at least one solid phase dispersed within at least one liquid phase.
2. State of the Art
Centrifugal separators are widely used for separating mixtures including constituents having different densities. Such devices have been found to provide a highly satisfactory method of separating mixtures comprising at least two or more insoluble liquids from one another. One particular colloidal suspension of interest is an emulsion, which is a mixture of two immiscible liquids (e.g., oil and water) in which one is colloidally suspended in the other.
Centrifugal separators, also referred to as extractors or contactors, may separate the individual constituents of a mixed input stream, provided that the constituents remain in separate phases and exhibit different densities. Typically, the liquid mixture may comprise a less dense phase (e.g., oil-based) and a heavier phase (e.g., water-based), which may be introduced into the centrifugal separator through an inlet that communicates with the interior of the centrifugal separator. The liquid mixture then enters the rotor of the centrifugal separator where centrifugal force separates the heavier phase from the lighter phase by forcing the heavier phase to flow outwardly away from the rotational axis of the rotor while displacing the lighter phase toward the rotational axis of the rotor.
The two phases are then individually collected at the upper end of the rotor with the heavier phase exiting at a location adjacent to the outer periphery of the rotor and the lighter phase exiting at a location adjacent to the rotational axis of the rotor. Typically, one or both of the exiting phases may be subjected to one or more subsequent stages of extraction, such as by circulation through another centrifugal separator.
One example of a method for centrifugally separating the components of a mixture is described in U.S. Pat. No. 4,959,158 to Meikrantz, the disclosure of which is incorporated, in its entirety, by reference herein. Also, U.S. Pat. No. 5,591,340 to Meikrantz et al., the disclosure of which is incorporated, in its entirety, by reference herein, discloses a centrifugal separator having a housing with a generally cylindrical inner surface defining an inner chamber. A hollow rotor is disposed within the chamber for rotation therein. At least one inlet is provided for introducing a liquid mixture into the annular volume between the rotor and the housing, where it is then directed into the rotor. An upper rotor assembly separates the liquid mixture by phase densities with the disparate constituents directed to respective outlets. In one embodiment of the invention, the upper rotor assembly includes a removable weir ring to facilitate “tuning” of the separation process. The rotor of the centrifugal separator is mounted on a unitary rotor shaft that extends axially through the separation chamber between upper and lower bearing assemblies in the separator housing. The bottom surface of the housing, where the liquid mixture is directed from the annular mixing volume into the rotor, preferably includes a plurality of radial vanes that are curved in the direction of rotation of the rotor to assist in directing the liquid mixture with minimal turbulence. Collector rings for the separated constituents provided from the upper rotor assembly are preferably formed integrally in the wall of the housing with a smoothly contoured peripheral surface to reduce turbulence of the output streams.
Enhanced separation of oil-water emulsions and dispersions using microwave radiation is disclosed in U.S. Pat. No. 4,582,629 to Wolf. In this disclosure, Wolf demonstrated through several benchtop experiments that microwave power applied to oil-water emulsions could increase oil-water separation rates by more than a factor of two compared to simple heating alone. Results suggested that microwaves were enhancing the separation rate through a mechanism distinct from heating alone. Additional disclosures relating to applying microwaves to oil-water emulsions include U.S. Pat. No. 4,853,507 to Samardzija; U.S. Pat. No. 5,055,180 to Klaila; U.S. Pat. No. 4,810,375 to Hudgins, et al.; and U.S. Pat. No. 4,853,119 to Wolf, et al. All of these patents relate to the advantages of enhanced emulsion breaking properties through the application of microwave radiation.
While generally applicable to any type of emulsion or suspension, one prevalent application of microwave-enhanced emulsion breaking technologies, for example, may be related to the petroleum industry. Another application for microwave-enhanced separation may include various applications related to food processing.
Regarding petroleum applications, crude oil pumped from wells may be typically co-mingled with suspended solids and water. Since the water and solids may be undesirable if contained in refinery feedstocks, it is preferable to remove these components. The separation of oil from water and solids using gravitational settling methods is typically incomplete and, therefore, unsatisfactory. The mixture which remains in such a process is a waste product and may consist of stable oil/water emulsions mixed with at least one solid phase.
It has been estimated that more than 2% of the crude oil currently pumped from the ground takes the form of these stable oil-water emulsions mixed with solids. Having little or no value to oil producers, the waste may typically be held in open pits and ponds or stored in large crude oil storage tanks. Such waste presents an ever-worsening remediation problem to oil producers and refiners.
Conventional methods to separate oil/water emulsions include application of heat, microbial breakdown, centrifugation, and chemical addition. However, most of these methods may not generate marketable product. Rather, only partial separation may be achieved and large amounts of waste that must be carefully disposed of may result. Also, conventional heating methods may exhibit problems with slow heat transfer into thick oil and water emulsions, accumulation of heavy layers of solid residue on heat transfer surfaces, and loss of valuable volatiles. Chemical demulsifiers, such as alum and polyamines, are available to break oil-water emulsions, but may be expensive or may pose difficulties in disposal. In addition, chemical treatment can be a relatively slow process that may not provide high levels of separation of particular emulsions.
Accordingly, microwaves may be relatively effective in facilitating crude-oil emulsion separation by heating the emulsion, since microwaves may penetrate deeply into the interior of thick or viscous emulsions, providing a very effective heating alternative. As the sludge is heated, viscosity is lowered, and rapid coalescence of liquid phases may occur. Generally, conventional apparatus and methods utilizing microwaves to enhance separation of a mixture may expose the mixture to microwave energy prior to introducing the mixture within a centrifugal separator.
One method and apparatus relating to exposing a material to microwave energy within a centrifuge are disclosed in U.S. Pat. No. 5,211,808 to Vilardi, et al., which relates to an apparatus for removal of water or other liquids and concentration of a substance. More particularly, Vilardi discloses a vacuum centrifugal concentrator for heating the substance being processed to enhance the concentration procedure.
U.S. Pat. No. 5,222,543 to Carlstrom, et al. discloses an apparatus for centrifugal casting of hollow articles. The apparatus has an elongate generally cylindrical hollow mold with a center conductor extending along the longitudinal axis so that the combination acts as a coaxial waveguide. Thus, a method for centrifugal casting, in which a casting composition is placed in a hollow rotating mold and microwave radiation is directed into the mold for heating thereof, is disclosed.
Microwaves within a centrifuge have also been used for cleaning. U.S. Pat. Nos. 5,344,493 and 5,368,171 to Jackson disclose the use of one or more dense fluids which are mixed with one or more chemical or physical agents and are simultaneously subjected to microwave radiation and centrifugal force to remove deeply recessed contaminants from internal and external surfaces of intricately arranged or formulated substrates. Subsequently, cleaned substrates are simultaneously subjected to microwave radiation and centrifugal force under vacuum to remove residual volatile contaminants. Additionally, the cleaned and sterilized substrates are contacted with chemical or physical agents to provide enhanced cleaning and to provide new and improved substrate properties. Finally, substrates which are prepackaged in semi-permeable membranes are cleaned using this apparatus, thus preventing recontamination of the cleaned substrates.
While the above-described conventional apparatus and methods relating to mixture separation may be useful for their intended purposes, it may be readily appreciated that it would be advantageous to provide improved methods and apparatus for centrifugal separation of a mixture comprising at least two constituents. For instance, a mixture may comprise a colloidal suspension or dispersion. Optionally or additionally, a mixture may comprise at least one solid phase.